The formation of laminated glass articles is broadly known in the art including the formation of laminated fibers by means of downward flow through concentric orifices as shown in U.S. Pat. Nos. 2,992,517 and 3,791,806. The formation of laminated sheet glass such as disclosed in U.S. Pat. Nos. 3,673,049 and 3,737,294 is also known, and includes the use of a laminated feeder orifice as shown in U.S. Pat No. 3,582,306, updraw lamination as shown in U.S. Pat. No. 3,218,143, and over-the-lip downdraw processes as shown in U.S. Pat. No. 1,734,964.
The reasons for developing a laminated sheet glass process include: improved strength, desired optical properties, cost reduction and weight reduction. Generally, the impetus for providing laminated sheet glass has been the inability to obtain a combination of desired properties in a single glass composition which is suitable for forming sheet glass for use in a desired end product.
Of major concern in the production of sheet glass, in addition to achieving necessary strength, is the obtainment of acceptable optical quality and thickness uniformity across the width thereof, which in the case of laminated sheet involves the thickness uniformity of each layer. Further, optical quality of the finished sheet glass is materially enhanced during the feeding and forming operations by providing free or virgin glass surfaces which are not in contact with the delivery or forming means, thus providing a virtually defect free optical quality surface. The concentric and laminating orifice discharge devices, on the other hand, being in direct contact with the glass flow, have the disadvantage of imparting boundary discontinuity or other defects to the surface of the laminated glasses flowing through such orifices, whereas similar surface discontinuities are also imparted to one surface of the over-the-lip delivered glass, which surface usually results in the outer optical surface when such glass is delivered to a forming member. Accordingly, such orifice and over-the-lip deliveries do not produce optimum optical quality. In addition, the updraw delivery, although providing good optical surface quality, does not provide the necessary means for achieving thickness uniformity of the various laminates across the width of the drawn sheet.
It thus has been an object of the present invention to provide a method and apparatus for laminating a plurality of glasses whose compositions impart specific attributes to the composite so formed and controlling the thickness of each glass layer across the width of the sheet while providing optical quality surfaces as formed.